Although ink supplies represent the second highest expense, after substrates (such as paper, cardboard, etc.), for the production of printed products, little care and efforts have been devoted so far to control the features of these ink supplies and their impact on production costs and quality. For instance, slight changes in ink formula can result in an important reduction of performance and/or efficiency, leading to substantial but hardly identifiable ink consumption and cost increases. For a large printing plant, such variations in ink quality may result in millions of dollars of extra costs annually. Furthermore, introduction of improper ink in a printing process may require stopping and cleaning of a press, which is highly troublesome and costly.
In an attempt to obviate the aforementioned drawbacks, some quality control is carried out on printed samples or on whole (unprinted wet state as supplied by manufacturers, or diluted, or bleached) ink samples. However, while evaluation of printed samples (generally by simple photometric measurements) produces a late indication of a problem and mainly addresses visual quality problems, testing whole ink samples prior to printing is typically carried out in laboratory through an extensive series of physical and mechanical tests, such as strength of ink, opacity, viscosity, etc., requiring fair amounts of ink and time. Furthermore, differences between the physical or mechanical properties of a sample with respect to reference values do not provide clear indication of the resulting impact on in-process performance of the tested ink.
For instance, U.S. Pat. No. 5,967,033, issued to Pfeiffer et al., on Oct. 19, 1999 discloses a method for determining ink coverage in a print image by analysis of an optical signal in the visible and near-infrared domain, reflected from a printed substrate. Similarly, in U.S. Pat. No. 4,935,628 (Martin et al.) issued on Jun. 19, 1990, ink from a writing instrument dried on a substrate is irradiated at multiple frequencies in the visible and infrared spectra, and the reflected signal is analysed for differentiation and authentication purposes by comparison with reference signals from a database. U.S. Pat. No. 6,275,285 granted to Nottke et al. on Aug. 14, 2001, also teaches a method for authentication of an ink sample dried on a substrate, but uses RAMAN spectrometry to obtain a higher level of resolution and discrimination of ink spectral signatures.
In U.S. Pat. No. 5,373,366 granted to Bowers on Dec. 13, 1994, concentration of a liquid ink sample is measured through illumination of the sample with a LED (Light Emitting Diode) and analysis of the direct and reflected optical signal using photodiodes. In a similar manner, Japanese Publication No. 60-202172A (Sato et al.) dated Dec. 10, 1985, discloses an ink production unit wherein liquid ink samples are analysed by UV/Visible spectrophotometry to provide indication of the density and appropriate feedback is supplied to the production unit for adjustment of the dilution rate. In U.S. Pat. No. 6,287,374 granted to Yanagida et al. on Sep. 11, 2001, wetting properties of a pigment in a water base liquid ink are measured by infrared spectrometry. In Japanese Publication No. 03-238345A, concentration of residual ink in paper pulp is measured by analysis of the signature of ink absorption in the near-infrared spectrum.
It should be mentioned that most of the above discussed prior technologies are concerned with jet-printing inks and writing instrument inks and that technologies used in connection with offset printing inks or the like are generally of the spectrophotometric type. Therefore, none of the above-discussed existing techniques provide an appropriate means for evaluating features and quality of a whole printing ink, and especially with regard to in-process performance. However, the above-mentioned references teach that infrared (IR) and near-infrared (NIR) spectrometry enable extended characterization of ink, providing sort of distinctive signature (also referred to as fingerprint). Such techniques also proved to be very effective for chemometric analysis of organic components such as resins, pigments or solvents found in media such as paints, dyes and inks, as well as for quality control in the pharmaceutical industry.
A few scientific publications confirm that Fourier Transform InfraRed (FT-IR) and Fourier Transform Near-InfraRed (FT-NIR) spectra of a liquid ink solution provide a unique signature, usable for authentication purposes. For instance, Rena A. Merrill and Edward G. Bartick stated in an article entitled “Analysis of Ballpoint Pen inks by Diffuse Reflectance Infrared Spectrometry” (Journal of Forensic Sciences, JFSCA, Vol. 37, No 2, March 1992, pp 528-541), that Diffuse Reflectance (DR) Fourier Transform Infrared Spectrometry (FT-IR) provides good results in matching spectra from ink solutions extracted from a questioned document with spectra from pure whole writing ink samples from a data base for identification purposes. The major cause of errors upon using this technique is related to the presence of substrate traces in the extracted ink solution.
Although the above examples show that methods exist to detect or evaluate ink properties, none of these methods is readily applicable to evaluation of a whole printing ink to reliably predict the in-process functional characteristics thereof and identify any features susceptible to negatively affect the performance of the ink(s) in a printing process such as offset, gravure, flexography, etc.
Thus a need exists for a technique that not only overcomes the limitations and drawbacks of the above-described methods, but that can be carried out using a very small sample from a whole printing ink supply, to provide indication of the degree of compliance with reference ink data and predict in-process performance characteristics prior to introduction of the ink into the actual process.